The present invention is directed to the removal of inks, particularly very fine particles of hydrophilic flexographic inks, from recycled paper products, i.e. deinking. Flexographic (hydrophilic) inks are becoming of greater and greater importance due to their greater environmental friendliness and lower equipment investment costs than when using conventional hydrophobic ink systems. However, limiting the growth in flexographic ink usage has been the difficulty in recycling paper with flexographic inks in flotation deinking systems.
"Deinking" is the process of removing ink and other contaminants from waste paper and there are two main techniques in current use. "Flotation deinking" entails forming an aqueous suspension of waste paper pulp fibers, inks, and other non-cellulosic contaminants and then mixing air into the suspension. In the presence of various additives, air bubbles selectively attach to ink particles and carry those particles to the surface of the aqueous suspension, thereby forming an ink rich froth. The froth is then removed leaving behind a relatively ink-free fiber slurry. Flotation deinking processes have heretofore been especially useful in removing hydrophobic inks with particle sizes larger than about 10 .mu.m. The additives used in such processes are generally specialty surfactants or fatty acids which are intended to agglomerate the relatively large hydrophobic ink particles to increase removal efficiency in the flotation cells. The presence of additives which would disperse the ink particles rather than agglomerate them is considered detrimental to the effectiveness of the flotation stage.
"Wash deinking," on the other hand, is particularly useful when the ink and other particles being removed are smaller than about 5 .mu.m. The process requires the addition of dispersants so that when a dilute waste paper pulp slurry is thickened, the very fine particles, including the hydrophilic flexographic type inks, will tend to stay with the water being removed to thereby produce a relatively clean pulp.
Newspapers, magazines and other printed media have been recycled for many years. Recently the need to recycle paper has increased significantly and will likely continue to increase in the future in view of environmental concern and legislative action. To reclaim fibers from printed material, a deinking process is required to remove the ink and other contaminants. Deinking of waste paper has become increasingly more difficult, however, because of changes in the printing techniques being used and the wide variety of printing inks. As a result, a slurry of recycled waste paper contain mixtures of hydrophilic and hydrophobic inks, resin binders, fillers, and the like, the substantial majority of which must be removed to produce a paper product having sufficient whiteness to be used in many commercial applications. The materials being removed, in addition to having a wide range of particle sizes, include polymeric binders such as polystyrene, polyacrylates, and acrylic copolymers. This has made removal of many of the newer inks more and more difficult. The industry response has generally been to utilize deinking systems which combine both flotation and washing processes.
However, the requirements for flotation and wash processes are different and are in conflict with each other. In flotation processes, relatively large ink particles are agglomerated to a size where they can be removed, while in washing processes relatively fine ink particles are dispersed so that they can be removed. This dichotomy has been further effected by recent changes in printing techniques which have resulted in an ever-increasing usage of hydrophilic flexographic inks. The very fine ink particles (&lt;5 .mu.m) associated with such flexographic inks can currently be substantially removed only by means of washing processes. The increasing usage of the very fine particle size hydrophilic flexographic inks is causing increasing problems in attaining the brightness necessary to permit the use of recycled paper products in many applications.
The difficulty in using flotation deinking for removal of hydrophilic flexographic inks is compounded because flotation deinking is generally operated under alkaline conditions, i.e. at a pH between about 8.5 and 10, while the hydrophilic flexographic inks become insoluble at pH's below about 7 so that neutral or acidic conditions are desired for their removal.
The conventional industry chemical formulations for flotation deinking for many years have entailed using a fatty acid or fatty acid soap. See, for example, U.S. Pat. Nos. 4,964,949 and 4,483,741. Fatty acids and fatty acid soaps have a number of problems associated with their use. They require high dosage rates, typically at least about 16 pounds/ton of waste paper (0.8 wt %) but often as high as 30 pounds/ton (1.5 wt %). Also, despite their use, fatty acids and fatty acid soaps are relatively poor foaming agents especially in the presence of hydrophilic flexographic inks which has led to current flotation deinking processes doing a poor performance of removing such inks.
In a Bulgarian laboratory study of flotation deinking in the early 1970's (prior to the advent of hydrophilic flexographic inks), waste paper containing hydrophobic inks dispersed in an asphalt binder was shredded, soaked in water for for 2-3 hr, fiberized and mixed with reagents to cause saponification of the asphalt binder and dissolution of the hydrophobic ink. Flotation deinking was performed with: liquid soap, technical grade oleic acid, stearic acid, tall oil fatty acids, and tall oil. Although few details or results are provided, the article states that the best results occurred when tall oil was used at an amount of 0.30%, with a flotation time of 15-20 minutes, and with the water exhibiting a hardness of less than about 3 mg eqv per liter (equivalent to 60 ppm Ca.sup.+2). In fact, the article states: "the hardness of the water used in the process must not exceed 3 mg eqv/liter." Mikhailova et al, "Deinking of Waste Newspapers by the Flotation Process," Tseluloza Khartiya 4, No. 2, 22-24, 30 (March/April 1973).
Tall oil generally contains (a) about 55-60 wt % tall oil fatty acids which have been reported to contain about 45 wt % oleic acid, about 40 wt % linoleic acid and about 15 wt % resin acid and (b) about 40-45 wt % rosin acids. Rosin acids (also known as resin acids) are alkylated, tricyclic, unsaturated organic acids. The two main rosin acids of tall oil are abietic acid and pimaric acid. Rosin acids and fatty acids are both commonly referred to as being amphipathic because they have a small polar carboxyl group attached to a large bulky hydrophobic hydrocarbon portion. When such acids are adsorbed onto the surface of a material, the polar hydrophilic end is oriented toward the surface of the material with the hydrophobic portion directed toward the water. Rosin acids are particularly useful as paper sizes. Commercial rosin acid products, generally containing more than about 80 wt % rosin acids, are also particularly useful as additives to provide tack (stickiness) for rubber polymers, in ink resin formulations, and in adhesive resins, particularly when color stability is not critical.
Recently, specific non-ionic surfactants such as are described in U.S. Pat. Nos. 4,964,949 and 5,100,574 have been especially developed for use in flotation deinking systems.
However, as the levels of hydrophilic flexographic ink continue to increase in waste paper, the previous deinking compositions are being found less and less effective in the removal of such ink particles in a flotation deinking procedure, especially one which is operated for about 8 minutes or less. As such, there is a continuing need to develop improved flotation deinking additive compositions which will effectively and rapidly deal with the changing compositions of waste recycled paper.
Accordingly, it is an object of the present invention to produce a deinked printed media which media had previously contained very fine hydrophilic ink particles, e.g. the flexographic inks present in increasing quantities in waste paper, by means of a flotation deinking process.
It is a further object of the invention to remove hydrophilic flexographic inks from printed media in a flotation deinking process while minimizing scale deposition therein.
It is a further object of the invention to remove hydrophilic flexographic inks from printed media in a flotation deinking process while limiting the time required for completing the process.
These and still further objects will be apparent from the ensuing description of the present invention.